Titanium nitride is a known barrier material used to prevent spiking of metal contacts, such as aluminum, into a silicon substrate. When aluminum is used as a contact material over a silicon substrate, subsequent heat treatment results in interdiffusion between the aluminum and the silicon, forming a conductive path between them that can short out devices. Titanium nitride is used as a barrier between aluminum and silicon.
Titanium nitride can be deposited into contact openings into which aluminum will be subsequently deposited by sputtering a titanium target in the presence of nitrogen and generally an inert gas. It is also known that treatment of a deposited titanium layer with oxygen improves the barrier properties of the titanium nitride layer. Sputtered titanium nitride forms a crystalline material, and spiking can still occur between the interstices of the crystals. Oxygen is adsorbed and fills these interstices, which is known as "stuffing" the titanium nitride. This stuffing has been done traditionally by using a furnace anneal, or heat treatment in a rapid thermal anneal (RTA) chamber with an oxygen-containing atmosphere. While this treatment reduces the spiking problem, both a sputtering chamber and an RTA chamber must be provided.
The adhesion of titanium nitride to silicon has also been improved by sputter depositing a first thin layer of titanium alone in the contact opening. This titanium layer can react with a silicon substrate to form titanium silicide, reducing the resistance of the contact.
A final thin layer of titanium can also be deposited as a wetting layer over the titanium nitride to improve adhesion of an overlying aluminum contact layer.
The deposition of these titanium-containing layers and the oxygen stuffing of the titanium nitride layer can all be carried out in a single sputtering chamber. A titanium target and an inert gas, such as argon, is used to sputter deposit the first layer of titanium; nitrogen gas is added to the chamber and sputtering is continued to form a titanium nitride layer. The oxygen stuffing treatment can be performed in the same chamber by flowing a mixture of oxygen and an inert gas and applying DC power to the target after the titanium nitride deposition.
The titanium nitride can be deposited in two modes; 1) the so-called poisoned mode, in which the titanium target is nitrided and titanium nitride is sputtered from the target; or 2) the metallic mode, in which the target is maintained as metallic titanium, and the titanium is sputtered from the target and reacts with nitrogen on the substrate surface, forming a titanium nitride film on the substrate. The poison mode titanium nitride is not as dense as the metallic mode titanium nitride.
Oxygen stuffing of the titanium layer of a Ti/TiN stack can be used as a barrier layer when titanium nitride is sputter deposited in the metallic mode sputtering method. However, when the titanium nitride is deposited in the poison mode method, the results are less than satisfactory and spiking of an overlying aluminum layer still occurs.
Thus the search for an improved process and improved contacts has continued.